Providing high-quality telephony services over packet networks has introduced many new technical challenges. One such challenge is to conceal channel erasures, which may occur due to packet loss. Normally, packet loss which is due to the late arrival of a given packet can be alleviated by using buffering techniques at the receiving terminal, at the expense of an increased end-to-end delay. Packet loss due to other causes can be mitigated by replacing missing segments with waveform segments based on correctly received packets. A number of such waveform substitution techniques (i.e., concealment techniques) have been proposed and will be familiar to those of ordinary skill in the art. Most of these techniques appear to be effective for short channel erasures (e.g., those less than about 20 milliseconds), but their performance drops quickly as the rate of channel erasure increases.
To improve a system's resilience to channel erasures, one well known approach is to employ multiple “uncorrelated” channels to deliver the same bit stream. Effectively, then, the channel is “erased” only when all channels fail on the same packet of information. Since all of these multiple channels are uncorrelated, the rate of channel erasure can be greatly reduced. This will in turn help to sustain the performance level of the aforementioned concealment techniques.
Such an improved communication system exploits the diversity from multiple uncorrelated channels to reduce the rate of channel erasure. However, there is no diversity in the encoded bit streams—the information received from more than one working channel will have no added value. A more advantageous result is achieved by sending different information over each channel in such a way that if the corresponding information from multiple channels are successfully transmitted, the information from each channel can be used to augment the information from other channels to thereby improve the overall fidelity of the reconstructed signal. On the other hand, if less than all of the channels are successfully transmitted, the information received will still be sufficient to achieve a reduced, but at least minimally acceptable fidelity. This approach, familiar to those skilled in the art, is known as multiple description (or multi-descriptive) source coding.
Recently, there have been extensive efforts dedicated to the design of efficient multiple descriptive coding systems. In particular, such systems typically allocate a separate codec (coder/decoder pair) for each channel, wherein each codec comprises a different encoder and a corresponding decoder. On the transmission side of the channels, the multiple encoders advantageously produce diversified information. At the receiving end, should one channel fail, the associated decoder temporarily stops its operation, and if necessary, may use conventional concealment mode techniques, fully familiar to those skilled in the art, to maintain any necessary internal memory states. Otherwise, each decoder operates normally. Output signals from all operating decoders are then mixed to produce the final decoded signal. (In the case where all channels have failed, a conventional concealment mode technique may be used to synthesize the output signal.)
Although the above-described scheme works well, the encoders and (more importantly) the decoders which are used must necessarily have been specifically designed for the given multiple description coding technique. Thus, existing network environments which employ encoders and decoders which have not been designed with multiple description coding applications in mind cannot take advantage of the benefits of such a technique. It would be extremely advantageous if the benefits of multiple description coding techniques could be realized in existing network environments, particularly without the need to modify the existing decoders, and preferably, with only slight modifications being required in the existing encoders.